Patent Application
20070057647
The present invention relates to the control and monitoring of single phase induction motors having an auxiliary start winding and a start capacitor in series with the start winding. The monitoring function may be used with permanent split capacitor (PSC) motors as well.
In single phase induction motors for applications requiring relatively high starting torques, it is quite common to utilize a start capacitor. In such motors, the start capacitor is initially connected to the start or auxiliary winding of the motor to enable a high starting torque to be developed. It is desirable that the start capacitor be disconnected as soon as the motor has started properly and before the high current through the start winding, and the stress on the start capacitor, can damage the winding and capacitor. It is empirically known that the conditions at which disconnection is desired, exist when the motor speed has increased to approximately 80 percent of synchronous speed.
Various electromechanical devices, such as a voltage responsive potential relay, have been employed for effecting disconnection of the start capacitor. Such relays are typically mounted remote from the motor or mounted in a suitable enclosure so as to enable them to be used in conjunction with hermetically sealed apparatus. However, these relays are not directly responsive to the motor speed. In a typical relay circuit arrangement, the relay coil is connected in parallel with the start winding, and the normally-closed relay contacts and start capacitor are connected in series with each other and in series with the parallel-connected start winding and relay coil. The relay coil is energized to effect opening of its contacts when the voltage across it, which is also the voltage across the start winding, reaches a predetermined pull-in value. It is desired that the predetermined pull-in value occur at the same approximately 80 percent of synchronous speed previously described. Typically, an appropriate relay must be selected that has a pull-in voltage value corresponding to the voltage at which the particular motor reaches 80 percent of synchronous speed. Thus, numerous relays must be made available for the various voltage levels at which different motors reach 80 percent of synchronous speed. In addition, because of variations, such as fluctuations in line voltage, the value of the voltage across the start winding at a specific motor speed will vary. Thus, when the value of the voltage across the start winding is the parameter chosen for effecting relay operation, the motor speed at which the start capacitor is disconnected can vary considerably.
Various embodiments of the present invention provide for control of single phase induction motors having an auxiliary start winding and at least a start capacitor in series with the start winding. One embodiment of the invention comprises an apparatus in combination with a single phase induction motor having a main winding, an auxiliary start winding and a start capacitor in series with the start winding. The apparatus comprises a sensing means for sensing the voltage across the main winding and across the auxiliary start winding of the induction motor, a means for detecting a condition of the sensed voltages across the main and auxiliary start windings that is indicative of the phase angle between the voltage across the main and auxiliary start windings, and a switching means for connecting the start capacitor in series with the auxiliary start winding when the switching means is de-energized, and for disconnecting the start capacitor from the auxiliary start winding when the switching means is energized. The apparatus further comprises a microcomputer for energizing the switching means to disconnect the start capacitor from the auxiliary start winding in response to determining when the phase angle between the voltage across the main winding and the voltage across the start winding increases by more than a predetermined amount, wherein the microcomputer de-energizes the switching means to re-connect the start capacitor to the auxiliary start winding in response to sensing a voltage condition across the auxiliary start winding indicative of a stall of the inductive motor.
In another aspect of the present invention, some embodiments of the invention comprise an apparatus that comprises a sensing means for sensing the voltage across the main winding and across the auxiliary start winding of the induction motor, a means for detecting a condition of the sensed voltages across the main and auxiliary start windings that is indicative of the phase angle between the voltage across the main and auxiliary start windings, and a microcomputer for monitoring the phase angle between the voltage across the main winding and the auxiliary start winding, and for responsively providing diagnostic information relating to the operation of the induction motor.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
The following description of the various embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
In one embodiment, an apparatus is provided for controlling a single phase capacitor-start, capacitor-run induction motor having a main winding and an auxiliary start winding and a start capacitor, as shown in
In one embodiment, the apparatus 30 comprises a switching means 32 for disconnecting the start capacitor 26 from the auxiliary start winding circuit. Specifically, the switching means 32 provides for connecting the start capacitor 26 in series with the auxiliary start winding 24 when the switching means 32 is de-energized, and for disconnecting the start capacitor 26 from the auxiliary start winding 24 when the switching means 32 is energized. The switching means in this embodiment is preferably a relay having normally closed contacts, but may alternately be a solid state switch such as a Triac.
In some embodiments, the apparatus 30 may comprise a first sensing means across nodes A and C for sensing a condition of the voltage across the main winding 22 of the induction motor 20, and a second sensing means across nodes B and C for sensing a condition of the voltage across the auxiliary start winding 24 of the induction motor 20. The first sensing means preferably comprises a voltage divider at 54 for sensing the voltage value across the main winding 22, but may alternately comprise a sensor for sensing a current value indicative of the sensed voltage across the main winding 22. Likewise, the second sensing means preferably comprises a voltage divider at 46 for sensing a voltage value representative of the voltage across the auxiliary start winding 24, but may alternately comprise a sensor for sensing a current value indicative of the sensed voltage across the auxiliary start winding 24.
An alternate voltage sensing implementation may use the voltage across the main and the capacitor voltage rather the to auxiliary winding voltage. Either implementation is related by the vector equation Vmain+Vauxiliary+Vcapacitor=0. In these embodiments the scalar values are of no interest. Only the direction or angle of the vectors provides useful information. The angles are related by this equation θ main+θ auxiliary+θ capacitor=180 degrees. The angle can be translated into the time domain by measuring either the zero voltage crossing point of the voltage vector of interest, or by measuring the logic state of the voltage vector after is passes through a fixed reference voltage level.
The apparatus 30 further comprises a microcomputer 38 that receives input (at 46 and 54) of the sensed voltages representative of the voltages across the main winding 22 and the auxiliary start winding 24, and determines a value that is indicative of the phase angle between the voltage across the main winding 22 and the auxiliary start winding 24. The microcomputer 38 responsively controls a triac 56 to energize the switching means 32 for disconnecting the start capacitor 26 from the auxiliary start winding circuit 24, in response to determining when the phase angle between the voltage across the main winding 22 and the voltage across the start winding 24 increases by more than a predetermined amount. The microcomputer 38 also comprises an output pin 58 for communicating diagnostic information relating to the operation of the motor 20.
Furthermore, motor operation can be monitored to determine if the motor is operating properly. Identification of system failure modes can provide active system protection to prevent damage and aid system troubleshooting and repair. Failure identification might include but is not limited to motor running properly, motor overloaded, motor stalled, motor protector tripped, main winding open, start winding open and capacitor open. Each failure mode may have a unique fault signature. Further areas of the controller 38 of the apparatus 30 of the present invention will become apparent from the detailed description of various embodiments provided hereinafter.
One embodiment of the apparatus 30 for controlling a single phase induction motor 20 is schematically shown in
After initial application of line voltage to the main winding 22, the voltage across the main winding 22 is periodically sensed at junction 54 by a first sensing means 50. In this embodiment, the first sensing means 50 is preferably a voltage divider circuit comprising resistors R4 and R5 across the main winding 22. The first sensing means 50 is preferably capable of periodically sensing the condition where the voltage across the main winding 22 has crossed zero volts and is increasing. When the first sensing means 50 detects this condition, the first sensing means provides an input to the microcomputer 38, which stores a time value indicative of when this condition in the voltage across the main winding 22 occurred. Likewise, the apparatus further comprises a second sensing means 52 that is preferably capable of periodically sensing voltage across the auxiliary start winding 24 at junction 46. In this embodiment, the second sensing means 52 is preferably a voltage divider circuit comprising resistors R2 and R3 across the auxiliary start winding 24. The second sensing means 52 senses the condition where the voltage across the auxiliary start winding 24 has crossed zero volts and is increasing. When the second sensing means 52 detects this condition, the second sensing means 52 provides an input to the microcomputer 38, which stores a time value indicative of when this condition in the voltage across the auxiliary start winding 22 occurred.
In one embodiment, the controller or microcomputer 38 is capable of monitoring these zero crossings of the voltage across the main and auxiliary start windings 22 and 24, and is preferably capable of monitoring the zero crossing point at every line cycle in the voltage across the main winding 22 and the auxiliary winding 24. The microcomputer 38 is further capable of determining a time difference value 40 from the sensed voltage conditions or zero cross points of the voltages across the main winding 22 and the auxiliary start winding 24. This time difference value 40 is indicative of the phase angle 60 between the voltage across the main winding 22 and the voltage across the auxiliary start winding 24. The microcomputer 38 is further capable of comparing the time difference value 40 between each periodically sensed zero cross occurrence in the voltage across the main and auxiliary start windings, with the immediately preceding time difference value between the preceding zero cross occurrence in the voltage across the main and auxiliary start windings. Thus, the microcomputer 38 is capable of determining from this time difference 40 whether the phase angle 60 between the voltage across the main winding 22 and the voltage across the auxiliary start winding 24 is increasing or decreasing, and at what rate.
Referring to
Curve 72 in
When start capacitor 26 is disconnected, the values of the torque and phase angle 60 change. Referring to
One embodiment of a method for controlling the apparatus for operating a single phase capacitor-start, capacitor-run induction motor having a main winding, an auxiliary start winding and a start capacitor is shown in
The method further provides for determining whether the phase angle 60 is increasing at step 130. If the sensed phase angle has not increased over the preceding sensed phase angle (and rather is decreasing while the motor speed is gradually increasing as shown in
As previously discussed, the motor 20 is at its maximum torque and is nearing 80 percent of its synchronous speed when the phase angle begins to increase rather than decrease, as determined at step 130. Thus, the method provides for energizing the relay at step 160 to cause the relay contacts to disconnect the start capacitor 26 from the auxiliary start winding 24, at such a time when the torque is sufficient to ensure that the motor 20 will not stall but rather will continue to run properly. The method further provides for checking whether there is a minimum phase angle present at step 140, indicating the presence of a bad start capacitor that is failing to provide a phase shift between the windings at step 145. The method also provides for verifying whether the phase angle 60 has changed more that a predetermined amount at step 150, indicating that the motor 20 has stalled at step 155.
Once the start capacitor 26 has been disconnected at step 160, the method monitors the phase angle 60 to verify whether the motor reaches full speed. If the phase angle is greater than 85 degrees at step 170 indicating that the motor has not reached full speed, the method checks at step 175 whether 120 line cycles have transpired since the relay was de-energized. If the motor does not reach full speed (as determined by a sensed phase angle of less than 85 degrees at step 170) within 120 line cycles at step 175, the method locks out the apparatus and provides a diagnostic communication indicating the motor is overloaded and cannot reach full speed after start up.
Once the start capacitor 26 has been disconnected at step 160 and the motor is operating normally, the method provides for monitoring the phase angle 60 between the voltage across the main and auxiliary start windings 22 and 24. If the motor 20 experiences a stall, the voltage in the auxiliary start winding will be lost or absent. Thus, the method is able to monitor the voltage sensing means in the auxiliary start winding for a voltage condition indicative of a stall at step 180. The method then provides for de-energizing the relay at step 190 to reconnect the start capacitor in series with the auxiliary start winding 24 to restart the motor 20.
In a second embodiment of the apparatus 30′, the switching means 32 is not energized by the apparatus 30′ but rather by the voltage level across the start winding 24 as shown in
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
